Apparatus and method for enhanced critical dimension scatterometry

ABSTRACT

Scatterometers and methods of using scatterometry to determine several parameters of periodic microstructures, pseudo-periodic structures, and other very small structures having features sizes as small as 100 nm or less. Several specific embodiments of the present invention are particularly useful in the semiconductor industry to determine the width, depth, line edge roughness, wall angle, film thickness, and many other parameters of the features formed in microprocessors, memory devices, and other semiconductor devices. The scatterometers and methods of the invention, however, are not limited to semiconductor applications and can be applied equally well in other applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Application No.60/656,712, filed on Feb. 25, 2005, which is incorporated by referenceherein.

TECHNICAL FIELD

The present invention is related to evaluating microstructures onworkpieces, such as semiconductor wafers, using apparatus and methodsthat can obtain a representation of the distribution of radiationreturning from the workpiece through a large range of angles ofincidence.

BACKGROUND

Semiconductor devices and other microelectronic devices are typicallymanufactured on a workpiece having a large number of individual dies(e.g., chips). Each wafer undergoes several different procedures toconstruct the switches, capacitors, conductive interconnects and othercomponents of a device. For example, a workpiece can be processed usinglithography, implanting, etching, deposition, planarization, annealing,and other procedures that are repeated to construct a high density offeatures. One aspect of manufacturing microelectronic devices isevaluating the workpieces to ensure that the microstructures are withinthe desired specifications.

Scatterometry is one technique for evaluating several parameters ofmicrostructures. With respect to semiconductor devices, scatterometry isused to evaluate film thickness, line spacing, trench depth, trenchwidth, and other aspects of microstructures. Many semiconductor wafers,for example, include gratings in the scribe lanes between the individualdies to provide a periodic structure that can be evaluated usingexisting scatterometry equipment. One existing scatterometry processincludes illuminating such periodic structures on a workpiece andobtaining a representation of the scattered radiation returning from theperiodic structure. The representation of return radiation is thenanalyzed to estimate one or more parameters of the microstructure.Several different scatterometers and methods have been developed forevaluating different aspects of microstructures and/or films ondifferent types of substrates.

Eldim Corporation of France manufactures devices that measure thephotometric and colorimetric characteristics of substrates used in flatpanel displays and other products. The Eldim devices use an OpticalFourier Transform (OFT) instrument having an illumination source, a beamsplitter aligned with the illumination source, and a first lens betweenthe beam splitter and the sample. The first lens focuses the light fromthe beam splitter to a spot size on the wafer throughout a large rangeof angles of incidence (e.g., (Φ=0° to 360° and Θ=0° to 88°). The lightreflects from the sample, and the first lens also focuses the reflectedlight in another plane. The system further includes an optical relaysystem to receive the reflected light and a sensor array to image thereflected light. International Publication No. WO 2005/026707 and U.S.Pat. Nos. 6,804,001; 6,556,284; 5,880,845; and 5,703,686 disclosevarious generations of scatterometers. The scatterometers set forth inthese patents are useful for assessing the photometric and colorimetricproperties of flat panel displays, but they may have several drawbacksfor assessing parameters of extremely small microstructures onmicroelectronic workpieces.

One challenge of scatterometry is properly locating very smallmicrostructures on a workpiece. This is not particularly difficult foranalyzing the pixels of a flat panel display because measuring thephotometric and colorimetric properties of such substrates merelyrequires locating the illumination spot on relatively large pixel areasinstead of very small periodic structures. As a result, systems used toanalyze flat panel displays may not include navigation systems capableof locating very small microstructures on the order of 20-40 μm.Moreover, the devices used to analyze flat panel displays may haverelatively large spot sizes that are not useful to measure theproperties of a 20-40 μm grating because such large spot sizes generatereflections from the surrounding areas that result in excessive noise.Therefore, devices designed for assessing flat panel displays may not bewell-suited for assessing gratings or other microstructures having muchsmaller dimensions on microelectronic workpieces.

Another challenge of using scatterometry to evaluate very smallmicrostructures is obtaining a useful representation of the radiationreturning from such microstructures. Existing scatterometers that assessthe films and surface conditions of flat panel displays typically userelatively long wavelengths of light (e.g., 532 nm). In contrast to flatpanel displays, many microstructures on semiconductor wafers have linewidths smaller than 70 nm, and such microstructures are continuallygetting smaller and being packed in higher densities. As a result, therelatively long wavelengths used to assess flat panel displays may notbe capable of assessing very small microstructures on manymicroelectronic devices. Therefore, devices used for assessing flatpanel displays may be further inadequate for assessing the properties ofmicrostructures on microelectronic workpieces.

Another challenge of assessing microstructures using scatterometry isprocessing the data in the representation of the return radiation. Manyscatterometers calculate simulated or modeled representations of thereturn radiation and then use an optimization regression to optimize thefit between the simulated representations and an actual reflectancesignal. Such optimization regressions require a significant amount ofprocessing time using high-power computers because the actualreflectance signals for measurements through a large range of incidenceangles contain a significant amount of data that is affected by a largenumber of variables. The computational time, for example, can requireseveral minutes such that the substrates are typically evaluated offlineinstead of being evaluated in-situ within a process tool. Therefore,many conventional scatterometers may not be well-suited for evaluatingmicrostructures on microelectronic workpieces.

Yet another challenge of assessing microstructures using scatterometryis calibrating the scatterometer. One difficulty of calibratingscatterometers is that the return radiation can have both p- ands-polarized components when the input path is off-axis relative to themicrofeature (e.g., a grating). This increases the complexity of fittingthe output to a model because the p- and s-polarized components must betreated separately. This is also challenging because the p- ands-polarized components change for each off-axis azimuth angle, and thusproper calibration requires measurements and calculations for severaldifferent azimuth angles in more sophisticated applications.

Calibrating scatterometers that operate over a large number of azimuthangles is also difficult because it is challenging to measure the p- ands-polarized components. One existing system for measuring p- ands-polarized components is a two-camera system that splits the outputbeam into separate p- and s-polarized beams which propagate at anon-parallel angle relative to each other. Such systems have one camerato detect the p-polarized component and another camera to detect thes-polarized component. The use of two cameras, however, is undesirablebecause the additional camera increases the cost and form factor of thescatterometer. This may prevent such two-camera scatterometers fromfitting into many integrated tool sets where metrology is desired.Additionally, it is time-consuming to calibrate two cameras because ofthe additional camera and compensating for the inherent variations inthe cameras. Such two-camera systems are also undesirable because theseparate images must be registered and integrated with each other toproduce a meaningful result. This is a significant, time-consumingcomputational procedure. Another system for measuring the p- ands-polarized components uses a single camera and a polarizer thatalternates between the p- and s-polarized components. This system mayhave problems because the serial presentation of the p- and s-polarizedcomponents to the detector requires more time to obtain themeasurements. Moreover, the polarizer is a mechanical device that movesbetween p- and s-polarizing states, and as such it may lack theprecision and accuracy to obtain meaningful measurements. Suchmechanical devices may wear out and further denigrate the precision andaccuracy of the calibration. Therefore, obtaining images of p- ands-polarized components for calibrating scatterometers or other usespresents a significant challenge in scatterometry.

Still another challenge of scatterometry is noise or inconsequentialdata in the measurements. In systems that are able to simultaneouslyobtain measurements through a large range of altitude and azimuthangles, the data in many areas of the resulting image may not bemeaningful. Therefore, there is a need to improve the process ofoperating scatterometers that simultaneously obtain measurements for alarge range of altitude and azimuth angles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a scatterometer in accordancewith an embodiment of the invention.

FIG. 2 is a schematic isometric view illustrating a portion of athree-dimensional convergence beam for irradiating microstructures on aworkpiece in accordance with an embodiment of the invention.

FIG. 3A is a schematic view illustrating an optical system for use in ascatterometer in accordance with an embodiment of the invention.

FIG. 3B is a schematic view of a cube-type polarizing beam splitter foruse in a scatterometer in accordance with an embodiment of theinvention.

FIG. 3C is a schematic view of a CMOS imager for use in a scatterometerin accordance with an embodiment of the invention.

FIG. 4 is a schematic view illustrating an optical system and anauto-focus system for use in a scatterometer in accordance with anembodiment of the invention.

FIG. 5A is a simulated radiation distribution for use in a scatterometerin accordance with an embodiment of the invention.

FIG. 5B is a measured radiation distribution provide by a scatterometerin accordance with an embodiment of the invention.

FIG. 6 is a schematic view illustrating a portion of a computer systemand a computational method for ascertaining parameters ofmicrostructures using a scatterometer in accordance with an embodimentof the invention.

FIG. 7 is a flow chart illustrating a method for determining one or moreparameters of a microfeature using a scatterometer in accordance with anembodiment of the invention.

FIG. 8 is a flow chart of a procedure for developing a predeterminedsensitivity record used in a method for assessing a parameter of amicrofeature on a workpiece in accordance with an embodiment of theinvention.

FIGS. 9A-9C are images illustrating additional aspects of developing apredetermined sensitivity record, acquiring a measured selectedradiation distribution, and fitting the measured selected radiationdistribution to a modeled selected radiation distribution.

DETAILED DESCRIPTION A. Overview

The present invention is directed toward evaluating microstructures onmicroelectronic workpieces and other types of substrates. Manyapplications of the present invention are directed toward scatterometersand methods of using scatterometry to determine several parameters ofperiodic microstructures, pseudo-periodic structures, and other verysmall structures having features sizes as small as 100 nm or less.Several specific embodiments of the present invention are particularlyuseful in the semiconductor industry to determine the width, depth, lineedge roughness, wall angle, film thickness, and many other parameters ofthe features formed in microprocessors, memory devices, and othersemiconductor devices. The scatterometers and methods of the invention,however, are not limited to semiconductor applications and can beapplied equally well in other applications.

One embodiment of the invention is directed toward a scatterometer forevaluating microstructures on workpieces. In this embodiment, thescatterometer comprises an irradiation source, a first optics assembly,and an object lens assembly. The irradiation source can be a laser thatproduces a first beam of radiation at a wavelength. The first opticsassembly is aligned with the path of the beam and configured tocondition the beam (e.g., shape, randomize, select order, diffuse,converge, diverge, collimate, etc.), and the object lens assembly ispositioned between the first optics assembly and a workpiece site. Theobject lens assembly is configured to focus the conditioned beam to aspot at an object focal plane. The object lens assembly or anotheroptical assembly of the scatterometer is also configured to (a) receiveradiation scattered from a workpiece and (b) present a distribution ofthe scattered radiation at a second focal plane. For example, theradiation distribution can be the intensity, polarimetric, ellipsometricand/or reflectance distribution of the scattered radiation. Thescatterometer of this embodiment can further include a detector, anavigation system, and an auto-focus system. The detector is positionedto receive at least a portion of the radiation distribution andconfigured to produce a representation of the radiation distribution.The navigation system is operatively coupled to the lens assembly or asupport structure holding the workpiece, and it is configured toidentify and locate the desired microstructure on the workpiece. Theauto-focus system is operatively coupled to one of the lens assembly orthe workpiece site, and it is configured to position the microstructureat a desired focal position (e.g., the object focal plane).

Another embodiment of a scatterometer in accordance with the inventioncomprises a laser configured to produce a beam of radiation having afirst wavelength, an optical system having a first optics assemblyconfigured to condition the beam of radiation, and a lens assembly. Thelens assembly is configured to focus the beam at an area of an objectfocal plane or other desired focal plane having a small spot size suchthat the beam has angles of incidence through a range of altitude anglesof at least approximately 0° to 45° and azimuth angles of at leastapproximately 0° to 90°. The altitude angle (Θ) is the angle between theillumination ray and a reference vector normal to the object focalplane, and the azimuth angle (Φ) is the angle between the incident planeand a reference vector in a plane parallel to the focal plane. The beammore preferably has angles of incidence through altitude angles of 0° togreater than 70° and azimuth angles of 0° to 360°. The scatterometer isfurther configured to collect and present the radiation scattered fromthe microstructure at a second focal plane. In one embodiment, the lensassembly itself presents the scattered radiation at the second focalplane, but in other embodiments the optical system has another opticmember that presents the radiation distribution at the second focalplane. The scatterometer of this invention further includes a detectorpositioned to receive the radiation distribution and configured toproduce a representation of the radiation distribution. Thescatterometer also includes a computer operatively coupled to thedetector to receive the representation of the radiation distribution.The computer includes a database and a computer-operable medium. Thedatabase has a plurality of simulated radiation distributionscorresponding to different sets of parameters of the microstructure. Thecomputer-operable medium contains instructions that cause the computerto identify a simulated radiation distribution that adequately fits therepresentation of the measured radiation distribution.

Another embodiment of the invention is a scatterometer for evaluating amicrostructure on a workpiece comprising an irradiation system, anoptical system, and a detector. The irradiation system includes a laserand/or a lamp, and the irradiation system is configured to produce afirst beam of radiation having a first wavelength and a second beam ofradiation having a second wavelength. The optical system has a firstunit configured to condition the first and second beams, and a secondunit configured to (a) focus the first and second beams at an area of anobject focal plane having a small spot size, and (b) present adistribution of scattered radiation returning from a microstructure at asecond focal plane. The detector is positioned to receive the radiationdistribution, and the detector is configured to produce a representationof the radiation distribution.

Another embodiment of a scatterometer in accordance with the inventioncomprises a laser configured to produce a beam of radiation having awavelength, an optical system, a detector, a calibration unit, and acomputer. The optical system has a first optics assembly configured tocondition the beam of radiation such that the beam is a diffuse andrandomized beam. The optical system also includes an object lensassembly configured to (a) focus the beam at an area of an object focalplane and (b) present scattered radiation returning from amicrostructure in a radiation distribution at a second focal plane. Thedetector is positioned to receive the radiation distribution of thescattered radiation and configured to produce a representation of theradiation distribution. One embodiment of the calibration unit includesa first calibration member having a first reflectivity of the wavelengthand a second calibration member having a second reflectivity differentthan the first reflectivity. The first and second calibration membersare located to be irradiated by the beam during a setup procedure todetermine a reference reflectance or other reference radiationdistribution. In other embodiments, the second calibration unit can beeliminated such that the second reflectance is from free space. Thecomputer is operatively coupled to the detector and includes acomputer-operable medium that determines the reference reflectance usinga first reflectance from the first calibration member and a secondreflectance from the second calibration member or free space.

Still another embodiment of a scatterometer in accordance with theinvention comprises a laser and/or lamp configured to produce a beam ofradiation having a wavelength and an optical system. The optical systemhas a first optics assembly including an object lens assembly configuredto focus the beam to an area at an object focal plane and present returnradiation scattered from a microstructure in a radiation distribution ata second focal plane. The optical system further includes a secondoptics assembly having a polarizing beam splitter configured to presentseparate images of p- and s-polarized components of the returnradiation. The scatterometer further includes a detector having a singlearray positioned to simultaneously receive the separate images of the p-and s-polarized components of the return radiation. The detector is alsoconfigured to produce a representation of the p- and s-polarizedcomponents of the return radiation.

Yet another embodiment of a scatterometer in accordance with theinvention comprises a laser and/or lamp configured to produce a beam ofradiation having a wavelength and an optical system having a firstoptics assembly. The first optics assembly includes an object lensassembly configured to focus the beam at an area on an object focalplane and present return radiation scattered from a microstructure in aradiation distribution at a second focal plane. The scatterometerfurther includes a detector comprising a CMOS imager have a die with animage sensor, focal optics, and packaging that defines an enclosedcompartment in which the focal optics and the image sensor are fixedwith respect to each other without a cover having parallel, flatsurfaces between the image sensor and the focal optics.

The present invention is also directed toward several methods forevaluating a microstructure on a workpiece. One embodiment of such amethod comprises generating a laser beam or a beam from a lamp having awavelength and irradiating a microstructure on a workpiece by passingthe beam through a lens assembly that focuses the beam to a focus areaat a focal plane. The focus area can have a dimension not greater than50 μm or in other embodiments approximately at least 10 of the periodicfeatures of the microstructure, and the beam simultaneously has altitudeangles of 0° to at least 15° and azimuth angles of 0° to greater than90°. In several applications, the focus area is not greater than 30 μm,and the altitude angles are 0° to about at least 45°. The altitudeangles can be from 0° to at least 70° in other examples. The methodfurther includes detecting an actual radiation distributioncorresponding to radiation scattered from the microstructure.

In another embodiment of a method in accordance with the invention theprocedure of irradiating a microstructure comprises irradiating thefocus area with a laser beam having a first wavelength and irradiatingthe focus area with a laser beam having a second wavelength differentthan the first wavelength. The first and second wavelengths can be in arange of approximately 200 nm-475 nm, and more specifically a firstwavelength can be from 200 nm-300 nm and a second wavelength can be from375 nm-475 nm. For example, the first wavelength can be approximately266 nm and the second wavelength can be approximately 405 nm, or inanother embodiment the first wavelength can be about 244 nm and thesecond wavelength about 457 nm. As such, the workpieces can beirradiated with one or more beams having one or more wavelengths lessthan 500 nm, but longer wavelengths may be used in other embodiments. Inparticular, a third wavelength of 633 nm may be used. Another aspect inaccordance with another embodiment of the invention includes calibratingthe detector by providing a first calibration member having a firstreflectivity and a second calibration member having a secondreflectivity. The system can be calibrated by determining a referencereflectance using a first reflectance from the first calibration memberand a second reflectance from the second calibration member. Otherembodiments can use only a single calibration member and obtain a secondreflectance measurement from free space.

In yet another embodiment of a method in accordance with the invention,the magnitude and phase of the scattered radiation is measured by thedetection system to perform an ellipsometric style measurement describedabove. The ellipsometric measurement in this embodiment can be performedby positioning polarizers, wave plates, or other phase modifying opticaldevices in the incident and/or detection optical system assemblies.

In yet another embodiment of a method in accordance with the invention,an automated workpiece transport system is incorporated with the opticalsystem and method to enable automatic high speed measurements acrossseveral workpieces without the need for moving or otherwise handling theworkpieces manually.

In yet another embodiment of a method in accordance with the invention,the optical system maintains a sine relationship between pixels on theimage detector and the altitude illumination angle theta. One example ofthis relationship is such that displacement, x, in the image planecorresponds to angle Θ so that x=F sin Θ, where F is some constant. Theadvantage to this implementation is that an adequate number of pixelscan be sampled throughout the entire image plane. More specifically, forcritical sampling of the image plane, the sampling frequency should betwice the highest spatial frequency in the plane. If the highest spatialfrequency does not depend on the position in the image plane, then thenumber of pixels to be averaged (to effectively make a larger pixel ofthe correct dimensions for critical sampling) does not depend on theposition in the image plane. For any other distribution, the number ofpixels required to be averaged will depend on position. Thus, unless asine relationship or another suitable relationship between the pixels onthe image sensor and the altitude angle is maintained, then the fixednumber of pixels available could result in some regions of the imageplane being sampled with fewer than the optimal number of pixels.

Various embodiments of the invention are described in this section toprovide specific details for a thorough understanding and enablingdescription of these embodiments. A person skilled in the art, however,will understand that the invention may be practiced without several ofthese details or additional details can be added to the invention.Well-known structures and functions have not been shown or described indetail to avoid unnecessarily obscuring the description of theembodiments of the invention. Where the context permits, singular orplural terms may also include the plural or singular term, respectively.Moreover, unless the word “or” is expressly limited to mean only asingle item exclusive from the other items in reference to a list of twoor more items, then the use of “or” in such a list is to be interpretedas including (a) any single item in the list, (b) all of the items inthe list, or (c) any combination of items in the list.

B. Embodiments of Scatterometers and Methods for EvaluatinqMicrostructures on Workpieces

FIG. 1 is a schematic illustration of a scatterometer 10 in accordancewith an embodiment of the invention. In this embodiment, thescatterometer 10 includes an irradiation source 100 that generates abeam 102 at a desired wavelength. The irradiation source 100 can be alaser system and/or lamp capable of producing (a) a beam 102 at a singlewavelength, (b) a plurality of beams at different wavelengths, or (c)any other output having a single wavelength or a plurality ofwavelengths. In many applications directed toward assessingmicrostructures on semiconductor workpieces, the irradiation source 100is a laser that produces a beam having a wavelength less than 500 nm,and more preferably in the range of approximately 266 nm-475 nm. Forexample, the wavelength can be about 375 nm-475 nm, or in some specificexamples about 405 nm or 457 nm. In a different embodiment, theirradiation source 100 can include a plurality of different lasersand/or filters to produce a first beam having a first wavelength ofapproximately 266 nm and a second beam having a second wavelength ofapproximately 405 nm, or in another embodiment the first beam can have awavelength of 405 nm and the second beam can have a wavelength of 457nm. It will be appreciated that the irradiation source 100 can produceadditional wavelengths having shorter or longer wavelengths in the UVspectrum, visible spectrum, and/or other suitable spectrum. Theirradiation source 100 can further include a fiber optic cable totransmit the beam 102 through a portion of the apparatus.

The scatterometer 10 further includes an optical system 200 between theirradiation source 100 and a workpiece W. In one embodiment, the opticalsystem 200 includes a first optics assembly 210 that conditions the beam102 to form a conditioned beam 212. The first optics assembly 210, forexample, can include a beam diffuser/randomizer that diffuses andrandomizes the radiation to reduce or eliminate the coherence of thebeam 102. The first optics assembly 210 can also include a beam elementthat shapes the beam to have a desired cross-sectional dimension, shape,and/or convergence-divergence. The beam element, for example, can shapethe beam 212 to have a circular, rectilinear, or other suitablecross-sectional shape for presentation to additional optic elementsdownstream from the first optics assembly 210.

The optical system 200 can further include an object lens assembly 300that focuses the conditioned beam 212 for presentation to the workpieceW and receives radiation reflected from the workpiece W. The object lensassembly 300 is configured to receive the conditioned beam 212 and forma convergent beam 310 focused at a discrete focus area S on a desiredfocal plane, such as an object focal plane 320. The convergent beam 310can have a conical shape when the conditioned beam 212 has a circularcross-section, but in other embodiments the convergent beam 310 can haveother shapes. For example, when the conditioned beam 212 has arectilinear cross-section, the convergent beam 310 has a pyramidalshape. As explained in more detail below with reference to Section C,the convergent beam 310 can have a range of incidence angles havingaltitude angles of 0° to greater than approximately 70° and azimuthangles of 0° to greater than 90° and more preferably 0-360°. Thealtitude angle is the angle between an incident ray and a referencevector normal to the object focal plane 320, and the azimuth angle isthe angle between an incident plane and a reference vector in a planeparallel to the object focal plane 320. The large range of incidenceangles generates a large number of unique data points that enableaccurate evaluations of several parameters of the microstructure.

The focus area at the object focal plane 320 preferably has a size andshape suitable for evaluating the particular microstructure. Forexample, when the microstructure is a grating or other structure on aworkpiece having a maximum dimension of approximately 10-40 μm, then thefocus area is also approximately 10-40 μm. The size of the focal area ispreferably not greater than the size of the microstructure so that theradiation does not reflect from features outside of the particularmicrostructure. In many applications, therefore, the object lensassembly 300 is configured to produce a spot size generally less than 40μm, and more preferably not greater than 30 μm. The scatterometer 10 canhave larger focus areas in other embodiments directed to assessinglarger structures.

The object lens assembly 300 is further configured to collect thescattered radiation reflecting or otherwise returning from the workpieceW and present the scattered radiation on a second focal plane 340. Theobject lens assembly 300, more particularly, presents the scatteredradiation in a manner that provides a radiation distribution of thescattered radiation at the second focal plane 340. In one embodiment,the object lens assembly 300 directs the scattered radiation coming atparticular angles from the object focal plane 320 to correspondingpoints on the second focal plane 340. Additional aspects of specificembodiments of the object lens assembly 300 are further described belowwith reference to Section C.

The optical system 200 can further include a beam splitter 220 throughwhich the conditioned beam 212 can pass to the object lens assembly 300and from which a portion of the return beam propagating away from thesecond focal plane 340 is split and redirected. The optical system 200can optionally include a second optics assembly 230 that receives thesplit portion of the return beam from the beam splitter 220. The secondoptics assembly 230 is configured to prepare the return beam for imagingby an imaging device. Additional aspects of specific embodiments of thesecond optics assembly 230 are described below with reference to SectionC.

The scatterometer 10 further includes a detector 400 positioned toreceive the radiation distribution propagating back from the secondfocal plane 340. The detector 400 can be a CCD array, CMOS imager, othersuitable cameras, or other suitable energy sensors for accuratelymeasuring the radiation distribution. The detector 400 is furtherconfigured to provide or otherwise generate a representation of theradiation distribution. For example, the representation of the radiationdistribution can be data stored in a database, an image suitable forrepresentation on a display, or other suitable characterizations of theradiation distribution. Several embodiments of the detector 400 aredescribed below in greater detail with reference to Section D.

The scatterometer 10 can further include a navigation system 500 and anauto-focus system 600. The navigation system 500 can include a lightsource 510 that illuminates a portion of the workpiece W and optics 520that view the workpiece W. As explained in more detail below, thenavigation system 500 can have a low magnification capability forlocating the general region of the microstructure on the workpiece(e.g., global alignment), and a high magnification capability forprecisely identifying the location of the microstructure. Severalembodiments of the navigation system can use the irradiation source 100and components of the optical system 200. The navigation system 500provides information to move the object lens assembly 300 and/or aworkpiece site 510 to accurately position the focus area of the objectlens assembly 300 at the desired microstructure on the workpiece W.

The auto-focus system 600 can include a focus array 610, and the opticalsystem 200 can include an optional beam splitter 240 that directsradiation returning from the workpiece W to the focus array 610. Theauto-focus system 600 is operatively coupled to the object lens assembly300 and/or the workpiece site 510 to accurately position themicrostructure on the workpiece W at the object focal plane 320 of theobject lens assembly 300 or another plane. As explained in more detailbelow with reference to Section E, the navigation system 500 and theauto-focus system 600 enable the scatterometer 10 to evaluate extremelysmall features of very small microstructures on semiconductor devices orother types of microelectronic devices.

The scatterometer 10 further includes a calibration system formonitoring the intensity of the beam 102 and maintaining the accuracy ofthe other components. The calibration system (a) monitors the intensity,phase, wavelength or other beam property of the beam 102 in real time,(b) provides an accurate reference reflectance for the detector 400 toensure the accuracy of the scatterometer, and/or (c) provides angularcalibration of the system. In one embodiment, the calibration systemincludes a detector 700 and a beam splitter 702 that directs a portionof the initial beam 102 to the detector 700. The detector 700 monitorschanges in the intensity of the beam 102 in real-time to continuouslymaintain the accuracy of the measured radiation distribution. Thedetector 700 can also or alternatively measure phase changes or adifferential intensity. The calibration system, for example, can use thepolarity of the return radiation to calibrate the system.

The calibration system can further include a calibration unit 704 havingone or more calibration members for calibrating the detector 400. In oneembodiment, the calibration unit 704 includes a first calibration member710 having a first reflectance of the wavelength of the beam and asecond calibration member 720 having a second reflectance of thewavelength of the beam. The first calibration member 710 can have a veryhigh reflectance, and the second calibration member 720 can have a verylow reflectance to provide two data points for calibrating the detector400. In another embodiment, the second calibration member 720 can beeliminated and the second reflectance can be measured from free space.

The scatterometer 10 further includes a computer 800 operatively coupledto several of the components. In one embodiment, the computer 800 iscoupled to the irradiation source 100, the detector 400, the navigationsystem 500, the auto-focus system 600, and the reference detector 700.The computer 800 is programmed to operate the irradiation source 100 toproduce at least a first beam having a first wavelength and preferablyto also produce a second beam having a second wavelength, as describedabove. The computer 800 can also control the source 100 to control theoutput intensity of the beam. The computer 800 further includes modulesto operate the navigation system 500 and auto-focus system 600 toaccurately position the focus area of the convergent beam 310 at adesired location on the wafer W and in precise focus.

In several embodiments, the computer 800 further includes acomputer-operable medium for processing the measured radiationdistribution to provide an evaluation of the microstructure on theworkpiece W. For example, the computer 800 can include a database havinga plurality of simulated radiation distributions corresponding to knownparameters of the microstructure. The computer 800 can includecomputer-operable media to process the measured radiation distributionin conjunction with the database of simulated radiation distributions ina manner that selects the simulated radiation distribution that bestfits the measured radiation distribution. Based upon the selectedsimulated radiation distribution, the computer stores and/or presentsthe parameters of the microstructure corresponding to those of thesimulated radiation distribution, or an extrapolation or interpolationof such parameters. In another embodiment, the computer 800 can scan orotherwise acquire data from pixels of the detector only where there is ahigh sensitivity to changes in the parameter(s). Such a selective inputto the computer reduces the amount of data and increases the quality ofthe data for processing in the computer 800. Several aspects of thecomputer 800 and methods for processing the measured radiationdistribution are set forth below in greater detail with reference toSection G.

C. Embodiments of Optics and Lens Assemblies

The scatterometer 10 can have several different embodiments of opticsassemblies and lens assemblies for optimizing the scatterometer for usewith specific types of microstructures. The object lens assembly 300,for example, can be achromatic to accommodate a plurality of beams atdifferent wavelengths, or it can have a plurality of individualassemblies of lenses that are each optimized for a specific wavelength.Such individual lens assemblies can be mounted on a turret that rotateseach lens assembly in the path of the beam according to the wavelengthof the particular beam, or such lenses may be mounted in separate, fixedpositions that correspond to the incident beam paths of the respectivewavelengths. In either case, the object lens assembly 300 is useful forapplications that use different wavelengths of radiation to obtaininformation regarding the radiation returning from the workpiece.

The object lens assembly 300 can also include reflective lenses that areuseful for laser beams in the UV spectrum. Certain types of glass mayfilter UV radiation. As such, when the beam has a short wavelength inthe UV spectrum, the object lens assembly 300 and other optic memberscan be formed from reflective materials that reflect the UV radiation.In another embodiment, the first optics assembly 210 or the object lensassembly 300 may have a polarizing lens that polarizes the radiation forthe convergent beam 310 (FIG. 1).

FIG. 2 illustrates one embodiment of the convergent beam 310 explainedabove with reference to FIG. 1 formed by an embodiment of the objectlens assembly 300. The convergent beam 310 illustrated in FIG. 2 has afrusto-conical configuration that results in a focus area S. The focusarea S is smaller than the area of the microstructure under evaluation,but it generally covers at least 8-10 of the periodic structures of themicrofeature. In several particular applications for the semiconductorindustry, the focus area S is approximately 10-40 μm in diameter, andmore preferably approximately 20-30 μm in diameter. The focus area S,however, is not limited to these ranges in other embodiments. The focusarea S may not necessarily be circular, and thus the convergent beam 310is typically configured such that the focus area S has a maximumdimension less than 30 μm (e.g., approximately 50 nm to approximately 30μm).

The convergent beam 310 simultaneously illuminates a microfeature Mthrough a wide range of incidence angles having large ranges of altitudeangles Θ and azimuth angles Φ. Each incidence angle has an altitudeangle Θ and an azimuth angle Φ. The object lens assembly is generallyconfigured to focus the beam to an area at the object focal planethrough at least (a) a 15° range of altitude angles and (b) a 90° rangeof azimuth angles simultaneously. For example, the incidence angles canbe simultaneously focused through altitude angles Θ of 0° to at least45°, and more preferably from 0° to greater than 70° (e.g., 0° to 88°),and azimuth angles Φ of 0° to greater than approximately 90°, and morepreferably throughout the entire range of 0° to 360°. As a result, theobject lens assembly 300 can form a conical beam having a large range ofincidence angles (Θ, Φ) to capture a significant amount of data in asingle measurement of the workpiece W. This is expected to enhance theutility and throughput of scatterometry for measuring criticaldimensions in submicron microstructures in real time and in-situ in aprocess tool.

FIG. 3 is a schematic diagram illustrating a specific embodiment of theoptical system 200 in accordance with the invention. In this embodiment,the first optics assembly 210 includes a beam conditioner 214 thatproduces a conditioned beam 212 including diffused and randomizedradiation. The beam conditioner 214 can be a fiber optic line thattransmits the beam from the irradiation source (not shown in FIG. 3) andan actuator that moves the fiber optic line to randomize the laser beam.The actuator can move the beam conditioner 214 in such a way that itdoes not repeat its movement over successive iterations to effectivelyrandomize the radiation.

The beam conditioner 214 can further include or alternatively be anorder sorter for removing undesired diffraction orders from the output.For example, the beam conditioner 214 may form a conditioned beam thatprovides a limited input to the object lens assembly 300 so that only asingle, specific diffraction illuminates pre-selected parts of thedetector. The beam conditioner 214 may include a carousel of aperturesplaced at the input of the optical system 200 so that different inputapertures may be selected according to the desired diffraction order ofthe conditioned beam 212.

The first optics assembly 210 can further include a field stop 216 andan illumination lens 218. The field stop 216 is positioned in the firstfocal plane of the illumination lens 218, and the field stop 216 canhave an aperture in a desired shape to influence the spot size and spotshape in conjunction with the illumination lens 218. In general, theillumination lens 218 collimates the radiation for presentation to theobject lens assembly 300.

The embodiment of the object lens 300 illustrated in FIG. 3 can includea plurality of separate lenses. For example, the object lens assembly300 can include a divergent lens 302, a first convergent lens 304, and asecond convergent lens 306. The first convergent lens 304 can have afirst maximum convergence angle, and the second convergent lens 306 canhave a second maximum convergence angle (see FIG. 4). In operation, theobject lens assembly 300 (a) focuses the conditioned beam 212 to formthe convergent beam 310 and (b) presents the return radiation from theworkpiece W on the second focal plane 340. The location of the secondfocal plane 340 depends upon the particular configurations of the lenses302, 304 and 306. For purposes of illustration, the second focal plane340 is shown as coinciding with the location of the first convergentlens 304.

The object lens assembly 300 is configured such that the angle (Θ_(x),Φ_(y)) of rays within the convergent beam 310 will pass throughcorresponding points (x, y) in the second focal plane 340. As a result,radiation passing through any given point (x, y) in the second focalplane 340 toward the workpiece W will pass through the object focalplane 320 at a particular corresponding angle (Θ_(x), Φ_(y)), andsimilarly radiation reflecting from the object focal plane 320 at aparticular angle (Θ_(x), Φ_(y)) will pass through a unique point (x, y)on the second focal plane 340. The reflected radiation passing throughthe second focal plane 340 propagates to the beam splitter 220 where itis directed toward the second optics assembly 230.

The second optics assembly 230 includes a relay lens 232, an output beamsplitter 234, and an image-forming lens 236. The relay lens 232 andoutput beam splitter 234 present the reflected and/or diffractedradiation (i.e., return radiation) from the beam splitter 220 to theimage-forming lens 236, and the image-forming lens 236 “maps” theangular distribution of reflectance and/or diffraction (i.e., theradiation distribution) from the second focal plane 340 to the imagingarray of the detector 400. In a particular embodiment, the image-forminglens 236 preferably presents the image to the detector 400 such that thepixels of the imager in the detector 400 can be mapped to correspondingareas in the second focal plane 340.

The second optics assembly 230 can further include a polarizing beamsplitter 238 to separate the return radiation into the p- ands-polarized components. In one embodiment, the polarizing beam splitter238 is positioned between the output beam splitter 234 and theimage-forming lens 236. In another embodiment, the beam splitter 238 ispositioned at a conjugate of the focal spot on the wafer along a pathbetween the image-forming lens 236 and the detector 400 (shown in dashedlines). In still another embodiment, the polarizing beam splitter 238can be located between the relay lens 232 and the output beam splitter234 (shown in dotted lines). The polarizing beam splitter 238 isgenerally located to maintain or improve the spatial resolution of theoriginal image of the focal spot on the workpiece. The location of thepolarizing beam splitter 238 can also be selected to minimize thealteration to the original optical path. It is expected that thelocations along the optical path between the relay lens 232 and theimage-forming lens 236 will be the desired locations for the polarizingbeam splitter 238.

The polarizing beam splitter 238 provides the separate p- ands-polarized components of the return radiation to improve thecalibration of the scatterometer 10 and/or provide additional data fordetermining the parameter(s) of the microfeature on the workpiece. Forexample, because the optics may perturb the polarization of the inputand output radiation, the polarizing beam splitter 238 provides theindividual p- and s-polarized components over the large range ofincidence angles. The individual p- and s-polarized components obtainedin this system can accordingly be used to calibrate the scatterometer 10to compensate for such perturbations caused by the optical elements.Additionally, the p- and s-polarized components can be used forobtaining additional data that can enhance the precision and accuracy ofprocessing the data.

FIG. 3B is a schematic view of a cube-type polarizing beam splitter foruse in the scatterometer 10 shown in FIGS. 2 and 3A. The cube-typepolarizing beam splitter 238 receives a return radiation beam 239 andsplits it into a p-polarized component beam 239 a and an s-polarizedcomponent beam 239 b. The cube-type polarizing beam splitter 238 can bea crystal with birefringence properties, such as calcite, KDP or quartz.The p- and s-polarized component beams 239 a-b exit from the cube-typepolarizing beam splitter 238 along at least substantially parallelpaths. The p- and s-polarized beams 239 a and 239 b are also spacedapart from each other such that they form separate images on thedetector 400. To increase the distance between the p- and s-polarizedcomponent beams 239 a-b, the size of the polarizing beam splitter 238can be increased. For example, as shown in dashed lines in FIG. 3B, alarger polarizing beam splitter 238 results in at least substantiallyparallel p- and s-polarized component beams 239 a-b that are spacedapart from each another by a larger distance than the polarizing beamsplitter 238 shown in solid lines 238. However, large cube-typepolarizing beam splitters can alter the p- and s-polarized beams, andthus the size of polarizing beam splitter 238 is generally limited. Aswith the non-polarized return radiation, the individual p- ands-polarized component beams 239 a-b impinge upon pixels of the detector400 in a manner that they can be mapped to corresponding areas in thesecond focal plane 340 shown in FIG. 3A.

One advantage of several embodiments of scatterometers includingcube-type polarizing beam splitters it that they provide fast,high-precision measurements of the p- and s-polarized components withgood accuracy. The system illustrated in FIGS. 3A-B use a single camerain the detector 400 to simultaneously measure both of the p- ands-polarized components of the return radiation 239. This systemeliminates the problems of properly calibrating two separate cameras andregistering the images from two separate cameras to process the datafrom the p- and s-polarized components. This system also eliminates theproblems associated with serially polarizing the return radiation beamusing a mechanically operated device because the polarizing beamsplitter 238 can be fixed relative to the return beam 239 and thedetector 400.

Another aspect of several embodiments of the optics is that a sinerelationship or another suitable relationship is maintained between thepixels on the image sensor and the altitude angles of the beam. Thisallows a linear relationship between pixels on the image sensor andaltitude angles. As such, the optics enable good sampling of the returnradiation even at the peripheral regions of an image sensor.

D. Embodiments of Detectors

The detector 400 can have several different embodiments depending uponthe particular application. In general, the detector is atwo-dimensional array of sensors, such as a CCD array, a CMOS imagerarray, or another suitable type of “camera” or energy sensor that canmeasure the intensity, color or other property of the scatteredradiation from the workpiece W corresponding to the distribution at thesecond focal plane 340. The detector 400 is preferably a CMOS imagerbecause it is possible to read data from only selected pixels with highrepeatability instead of having to read data from an entire frame. Thisenables localized or selected data reading, which is expected to (a)reduce the amount of data that needs to be processed and (b) eliminatedata that does not have a meaningful contrast. Additional aspects ofusing CMOS images for image processing are described in more detailbelow. The p- or s-polarized components can be measured with a singleCMOS imager to determine certain characteristics that are otherwiseundetectable from non-polarized light. As such, using a CMOS imager andpolarizing the reflected radiation can optimize the response to increasethe resolution and accuracy of the scatterometer 10.

FIG. 3C is a schematic view showing a CMOS imager assembly for use inthe detector 400 in accordance with an embodiment of the invention. Inthis example, the CMOS imager assembly includes a die 410 having animage sensor 412, focal optics 420, and packaging 430 defining anenclosed compartment 432 between the die 410 and the focal optics 420.The focal optics 420 typically have curved surfaces or otherconfigurations such that they are not merely a plate having parallel,flat surfaces. Additionally, the CMOS imager assembly does not have aglass cover or other optical member with parallel, flat surfaces betweenthe image sensor 412 and the focal optics 420. As such, the CMOS imagerassembly illustrated in FIG. 3C does not have any flat optics in thecompartment 432 between the image sensor 412 and the focal optics 420.In this embodiment, the polarizing beam splitter 238 is just upstream ofthe CMOS imager assembly 400 relative to the return radiation beam 239.

The CMOS imager assembly 400 illustrated in FIG. 3C is expected toprovide several advantages for use in scatterometers. In severalembodiments, for example, the lack of a cover or other flat opticalmember between the image sensor 412 and the focal optics 420 is expectedto reduce perturbations in the return radiation beam 239 at the imagesensor 412. More specifically, a glass member with parallel, flatsurfaces between the focal optics 420 and the image sensor 412 can alterthe return radiation just before it reaches the image sensor 412. Byeliminating such glass members with parallel, flat surfaces, the CMOSimager assembly illustrated in FIG. 3C is expected to eliminatedistortion or interference caused by a glass member with parallelsurfaces.

E. Navigation and Auto-Focus Systems

Referring back to FIG. 1, the navigation system 500 accurately alignsthe beam 310 with a desired area on the workpiece W, and the auto-focussystem 600 adjusts the object lens assembly 300 or workpiece site 510 sothat the object focal plane 320 is at the microstructure. In oneembodiment, the navigation system 500 has a separate illuminationsource, lens and measurement optics for determining the precise locationof the microstructure on the workpiece W. The light source of thenavigation system 500 can be a LED, and the lens and optics can be atwo-stage system having low and high magnifications. The lowmagnification stage identifies the general area on the wafer where themicrostructure is located, and the high magnification stage refines thelocation. In other embodiments, the navigation system 500 can includeadditional relay optics introduced to image the surface directly throughthe object lens assembly 300.

The auto-focus system 600 can be a camera correlation focus systemhaving a dihedral mirror that simultaneously splits the illuminationpupil in two and redirects the light from the two halves of the dihedralmirror to different sections of a CCD array. The displacement betweenthe two images is used to automatically determine the focus. A fieldstop can be incorporated to prevent overlap of the two images on thefocus camera. The field stop is included in the illumination beam of themicroscope of the auto-focus system.

FIG. 4 is a schematic illustration of an embodiment of the navigationsystem 500 and auto-focus system 600 for use in the scatterometer.Several aspects of FIG. 4 are similar to those explained above withreference to FIGS. 1 and 3A, and thus like reference numbers refer tolike components in these figures. The navigation system 500 can have ahigh magnification system associated with the metrology system. Forexample, the high magnification system includes a light source 550, suchas an LED, that injects light via a beam splitter 552 and is focused onthe second focal plane by a relay lens 553 via beam splitter 240. Thislight illuminates the workpiece and is reflected back through the objectlens assembly 300. The reflected light is directed by beam splitter 220and through lenses 232 and 554 to camera 560. The lenses 232 and 554form an image of the microstructure on the camera 560.

The auto-focus system 600 in this embodiment shares the relay lens 553and the beam splitter 552 with the navigation system. The beam splitter552 directs a beam 620 to a dihedral mirror 630, an image lens 632, anda steering mirror 634. The first beam 620 is then received by anauto-focus detector 640, such as a CCD array or other type of camera.

F. Calibration

The calibration system is used to monitor the properties of the initialbeam 102 (FIG. 1) and calibrate the system efficiency for accuratelydetecting the radiation distribution. The beam properties are monitoredby a reference detector 700 that receives a portion of the beam 102 inreal time. As the beam fluctuates, the reference detector 700 detectsthe changes in the beam 102 and sends a signal to the computer 800. Thecomputer 800 accordingly normalizes and/or performs other computationaloperations to the measured intensities, or it adjusts the measuredradiation distribution by the variances in the intensity of the initialbeam 102, to compensate for small changes in the beam 102. Unlike somesystems that do this periodically, the computer 800 continuouslyreceives signals from the reference detector 700 to maintain theaccuracy of the system in real time. This is expected to significantlyenhance the accuracy and precision with which the scatterometer 10 canevaluate extremely small features in microstructures.

The calibration system can also include a calibration unit, such as thecalibration unit 704 (FIG. 1) with one or more calibration members, forproviding photometric calibration of the system. In one embodiment, thefirst calibration member 710 can be a highly reflective mirror having areflectance greater than 95%, and more preferably a reflectance ofapproximately 99.99%. The first calibration member 710 can be configuredto have a consistent reflectance through a wide range of altitudeangles. The second calibration member 720 can be black glass having alow reflectance (e.g., 0% to 10%). In operation, the detector 400 iscalibrated by measuring the reflectance of the beam from the firstcalibration member 710 and from the second calibration member 720 toprovide two data points corresponding to the known 99.99% reflectance ofthe first calibration member 710 and the known 0% reflectance of thesecond calibration member 720. Using these two data points, a straightline can be obtained to provide a reference reflectance of the detector400.

In another embodiment of the calibration unit 704, the secondcalibration member 720 is not included or the second calibration membercan be free space such that there is no reflectance of the illuminationradiation. In this embodiment, the scatterometer 10 is calibrated byobtaining a first reflectivity from the first calibration member 710 anda second reflectivity from an area separate from the first calibrationmember 710. When the second reflectivity is obtained from free space,there is approximately zero percent reflectance such that a straightline can be obtained from these two measurements to provide a referencereflectance of the detector 400.

The scatterometer can be calibrated further using several differentmethods. For example, a known grating with a known radiationdistribution can be measured using the scatterometer 10 to determinewhether the detector 400 accurately produces a representation of theradiation distribution. In another embodiment, a thin film having aknown thickness can be irradiated to determine whether the detector 400provides an accurate representation of the radiation distribution fromsuch a thin film. Both of these techniques can also be combined for yetanother calibration method.

G. Computational Analyses

The computer 800 can use several different processes for determining oneor more parameters of the microstructure based on the measured radiationdistribution from the detector 400. In general, the computer 800compares the measured radiation distribution with one or more simulatedradiation distributions corresponding to selected parameters of thefeatures and materials of the microstructure (e.g., height, width, lineedge roughness, roundness of edge corners, spacing, film thickness,refraction index, reflection index, and/or other physical properties).Based on the comparison, the computer 800 then stores and/or provides anoutput of one or more parameters of the microstructure.

FIG. 5A is an image illustrating a simulated radiation distribution 810having a first interference pattern 812 including a plurality of thinarcs, a second interference pattern 814 including a plurality ofdifferent arcs, and a third interference pattern 816 in a configurationof a “bulls-eye.” The first interference pattern 812 can correspond tothe specular reflections, the second interference pattern 814 cancorrespond to higher order diffractions, and the third interferencepattern 816 can correspond to the film thickness. The symmetry of theimage can also be assessed to provide additional information regardingthe microstructure. For example, in overlay applications, asymmetry inthe image can be used to evaluate the skew between overlay structures(e.g., the extent of misregistration). Another example of using theimage symmetry is determining the asymmetry of sidewall angles ofgrating lines. The interference patterns of the simulated radiationdistribution 810 are unique to each set of feature parameters, and thuschanging one or more of the feature parameters will produce a differentsimulated radiation distribution.

FIG. 5B is an image of a measured radiation distribution 820 of anactual microstructure on a workpiece. The measured radiationdistribution 820 includes a corresponding first interference pattern822, a second interference pattern 824, and a third interference pattern826. In operation, the computer 800 ascertains the parameters of themicrostructure by selecting and/or determining a simulated radiationdistribution 810 that best fits the measured radiation distribution 820.

FIG. 6 illustrates one embodiment for ascertaining the featureparameters of the microstructure. In this embodiment, the computer 800includes a database.830 including a large number of predeterminedsimulated reference radiation distributions 832 corresponding todifferent sets of feature parameters. The computer 800 further includesa computer-operable medium 840 that contains instructions that cause thecomputer 800 to select a simulated radiation distribution 832 from thedatabase 830 that adequately fits a measured radiation distribution 850within a desired tolerance. The computer-operable medium 840 can besoftware and/or hardware that evaluates the fit between the storedsimulated radiation distributions 832 and the measured radiationdistribution 850 in a manner that quickly selects the simulatedradiation distribution 832 having the best fit with the measuredradiation distribution 850 or at least having an adequate fit within apredetermined tolerance. In the case where a plurality of the simulatedradiation distributions 832 have an adequate fit with the measuredradiation distribution 850, the computer 800 can extrapolate orinterpolate between the simulated distributions. Once the computer hasselected a simulated radiation distribution with an adequate fit or thebest fit, the computer selects the feature parameters associated withthe selected simulated distribution.

In an alternative embodiment, the computer calculates a simulatedradiation distribution and performs a regression optimization to bestfit the measured radiation distribution with the simulated radiationdistribution in real time. Although such regressions are widely used,they are time consuming and they may not reach a desired result becausethe regression may not converge to within a desired tolerance.

In still other embodiments, the computer 800 may perform furtherprocessing or different processing such as finite element models forevaluating non-periodic or pseudo-periodic structures. The computer 800may also be-able to solve for the refraction index and reflectivityindex of the particular materials by determining the film thickness.Therefore, the enhanced data in the measured radiation distributionenables the computer 800 to more accurately determine the featureparameters of the microstructure and may enable more feature structuresto be monitored (e.g., line edge roughness, refraction index,reflectivity index, etc.).

FIG. 7 is a flow chart showing a method 900 for ascertaining one or moreparameters of a microstructure using the computer 800 in accordance withanother embodiment of the invention. In this embodiment, the method 900includes a first stage 910 comprising irradiating a microstructure on aworkpiece by passing a beam through an object lens assembly thatsimultaneously focuses the beam to a focus area at an object planethrough a large number of incidence angles. The beam for example, can befocused simultaneously through angles of incidence having altitudeangles of 0° to at least 45° and azimuth angles of 0° to greater than90°. The method 900 further includes a second stage 920 comprisingobtaining an actual radiation distribution of scattered light or otherradiation returning from the microstructure through the angles ofincidence. The actual radiation distribution can be obtained using anarray of addressable pixels that can be scanned or read individually.The method 900 further includes a third stage 930 comprising acquiring ameasured selected radiation distribution by reading data from selectedpixels in the array that have sufficient sensitivity to changes in theparameter based upon a predetermined sensitivity record, and a forthstage 940 comprising fitting the measured selected radiationdistribution to simulated or modeled selected radiation distributionscorresponding to the selected pixels to determine a value of theparameter. The method 900 is expected to be particularly useful becauseselected areas of the pixel array with high sensitivity to changes inthe measured parameter can be scanned, and then this smaller amount ofdata that is more sensitive to changes in the measured parameter can befitted to modeled distributions.

The first stage 910 of the method 900 can be performed using ascatterometer as described and shown above with reference to FIGS. 1-4.As such, an input beam can be passed through an object lens assemblythat forms a beam having a large range of incident angles (Θ, Φ) tocapture a significant amount of data in a single measurement of theworkpiece. The altitude angles Φ can be from 0° to approximately 80° to88° and the azimuth angles Θ can be from 0° to 360° as explained above.

After irradiating the microstructure, the second stage 920 of the method900 can include obtaining the actual radiation distribution of radiationreturning from the microstructure through the same angles of incidenceas described above with reference to the detector 400. In thisembodiment, the detector is preferably a CMOS imager that has an arrayof addressable pixels in which individual pixels can be independentlyscanned.

The third stage 930 of the method 900 comprises scanning or otherwiseacquiring the actual intensity measurements from selected pixels of thearray corresponding to angles of incidence that are highly sensitive tochanges in the measured parameter(s) based upon a predeterminedsensitivity record. The sensitivity record can be a pixel-by-pixelanalysis of a plurality of pixels in the array that correspond toindividual incidence angles (Θ_(n), Φ_(n)). At each pixel, modelintensities of the return radiation are calculated for different valuesof the parameter, and then the model intensities are subtracted fromeach other to determine the magnitudes of the changes in the intensityfor the incremental changes in the parameter. Larger intensity changescorrespond to pixels and angles of incidence that are more sensitive tochanges in the parameter compared to smaller changes in the intensity.

FIG. 8 is a flow chart illustrating an embodiment of the third stage 930for use in the method 900. In this embodiment, the third stage 930includes identifying a pixel of the array (procedure 932), and thencalculating model intensities of the return radiation at the pixel thatcorrespond to different values of the parameter (procedure 934). Thisembodiment of the third stage 930 further includes differencing themodel intensities (procedure 936) to determine the magnitude of changein the intensities corresponding to changes in the parameter. Based uponthe magnitudes of the changes in the model intensities determined inprocedure 936, the third stage 930 further includes assigning asensitivity value (procedure 938) to a corresponding pixel. A thirdstage 930 can be repeated for any number of pixels in a CMOS imagerarray to develop a predetermined sensitivity record that associates thepixels in the array with the sensitivity to changes in the parameter.

FIGS. 9A and 9B graphically illustrate an embodiment of developing apredetermined sensitivity record for a parameter and selecting pixels inthe array that have sufficient sensitivity to changes in the parameterfor use in the measured selected radiation distribution. FIG. 9A, morespecifically, shows model intensity distributions for three differentvalues of a parameter (e.g., the critical dimension) through angles ofincidence where 0° is from 0° to approximately 65° and Φ is from 0° toapproximately 90°. In FIG. 9A, the intensity distribution 952corresponds to a first value of the parameter, the intensitydistribution 954 corresponds to a second value of the parameter, and theintensity distribution 956 corresponds to a third value of theparameter. Although the data in the intensity distributions 952, 954 and956 is useful, it does not provide an indication of which incidenceangles (e.g., pixels in a CMOS imager) are more sensitive to changes inthe parameter. FIG. 9B is a sensitivity map 960 illustrating asensitivity record associating the sensitivity of various incidenceangles to changes in the parameter. The sensitivity map is obtained bydifferencing the model intensities at corresponding incidence anglesbetween the intensity distributions 952, 954 and 956 shown in FIG. 9A.The sensitivity map 960 shown in FIG. 9B assigns a sensitivity valueaccording to the magnitude of the difference between the model intensitydistributions. In this embodiment, bright regions indicate angles ofincidence that are more sensitive to changes in the parameter and darkregions indicate angles of incidence that are less sensitive to changesin the parameter. The sensitivity values for the angles of incidence canthen be associated with the pixels in the CMOS imager arraycorresponding to the angles of incidence.

One aspect of the method 900 is that the sensitivity map 960, which is agraphical representation of the sensitivity record, provides anindication of the angles of incidence that may provide the most valuabledata corresponding to changes in the parameter. Additionally, becausespecific pixels of a CMOS imager can be scanned or read individually,one aspect of the method 900 is determining which pixels in the CMOSimager array have high sensitivities from which the measured intensityvalues can be acquired (e.g., scanned).

FIG. 9C illustrates one embodiment of selecting pixels in the array thathave sufficient sensitivity to changes in the parameter based upon thepredetermined sensitivity record or sensitivity map. Referring to FIG.9C, pixels in the CMOS imager array corresponding to angles of incidencein the bright region 962 are expected to provide good data because thisarea indicates a region of high sensitivity, and pixels associated withangles of incidence in the dark region 964 are likely to have lowsensitivity to changes in the parameter. As such, the measured selectedradiation distribution can be acquired by reading data from the pixelsin the array corresponding to angles of incidence that have a highsensitivity to changes in the parameter based on the predeterminedsensitivity record. After scanning the pixels that correspond to anglesof incidence which have a high sensitivity, the measured selectedradiation distribution from such pixels is compared to modeled radiationdistributions from the same angles of incidence until the measuredselected radiation distribution adequately fits with one of the modeledselected radiation distributions. At this point, a value of theparameter is determined according to the corresponding best fit of themodeled selected radiation distribution.

The method 900 can be applied to applications in which severalparameters of the microfeature are to be assessed using scatterometry.In such multi-parameter applications, separate sensitivity records areestablished for individual parameters by varying the parameter ofinterest while keeping the other parameters constant. The regions of theCMOS imager array that are scanned can then be determined by selectingthe sets of pixels corresponding to high sensitivity values for theparameters that are to be assessed using the scatterometer.

In practice, the sensitivity record can be used to optimize the scanpath to retrieve data from only the pixels that have sufficientsensitivity, and to also fit the data from the selected pixels tocorresponding models in a library. This is expected to significantlyreduce the simulation times for arriving at a value of one or moreparameters because it reduces the number of (Θ, Φ) combinations thatneed to be stored in a library and used to fit the measured selectedradiation distribution to a modeled selected radiation distribution. Thesystem is also expected to increase the acquisition rate because only aportion of the pixels in the CMOS imager need to be scanned for eachmeasurement. This provides more measurements in a given exposure period,which can lead to better averaging and lower signal-to-noise ratios.This method is also expected to enhance the precision (e.g.,repeatability) because it uses only data from highly sensitive pixels.Additionally, this method is useful because it is possible to overlayknown pixel noise for a particular CMOS imager array with thesensitivity array such that pixels with high sensitivities caused bynoise can be eliminated from the measurements. Therefore, such an imagefitting procedure that scans only selected pixels of a CMOS imager arraywith high sensitivity values is expected to significantly improve thecomputational analysis for determining values of parameters inscatterometry.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thespirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A scatterometer for evaluating microstructures on workpieces, comprising: a laser and/or lamp configured to produce a beam of radiation having a wavelength; an optical system having (i) a first optics assembly including an object lens assembly configured to (a) focus the beam in an area at an object focal plane, and (b) present return radiation scattered from a microstructure in a radiation distribution on a second focal plane, and (ii) a second optics assembly having a polarizing beam splitter configured to present separate images of p- and s-polarized components of the return radiation; and a detector having a single array positioned to receive the separated images of the p- and s-polarized components of the return radiation and configured to produce a representation of the p- and s-polarized components of the return radiation, wherein the detector and polarizing beam splitter are configured such that the detector receives the separated images of the p- and s-polarized components simultaneously.
 2. The scatterometer of claim 1 wherein the detector comprises a CMOS imager having an array of pixels, and wherein the p-polarized component of the return radiation impinges upon a first region of the array and the s-polarized component of the return radiation impinges simultaneously upon a second region of the array.
 3. The scatterometer of claim 1 wherein the polarizing beam splitter comprises a cube-type polarizing beam splitter.
 4. The scatterometer of claim 1 wherein the detector comprises a CMOS imager having an array of pixels and the polarizing beam splitter comprises a cube-type polarizing beam splitter that directs the p- and s-polarized components of the return radiation along parallel paths such that the p-polarized component of the return radiation impinges upon a first region of the array and the s-polarized component of the return radiation impinges simultaneously upon a second region of the array.
 5. The scatterometer of claim 1 wherein the object lens assembly is further configured to simultaneously focus the conditioned beam at the object focal plane through at least (a) a 15° range of altitude angles and (b) a 90° range of azimuth angles.
 6. The scatterometer of claim 5 wherein the altitude angles are 0° to at least 70° and the azimuth angles are 0° to at least 180°.
 7. The scatterometer of claim 5 wherein the altitude angles are 0° to at least 80° and the azimuth angles are 0° to at least 360°.
 8. The scatterometer of claim 1 wherein the wavelength is approximately 200 nm to approximately 475 nm.
 9. The scatterometer of claim 1 wherein the wavelength is approximately 375 nm to approximately 475 nm.
 10. The scatterometer of claim 1 wherein the first wavelength is approximately 244 nm and the second wavelength is approximately 457 nm.
 11. The scatterometer of claim 1 wherein the wavelength is approximately one of 405 nm or 457 nm.
 12. The scatterometer of claim 1 wherein: the detector comprises a CMOS imager having an array of pixels and the polarizing beam splitter comprises a cube-type polarizing beam splitter that directs the p- and s-polarized components of the return radiation along parallel paths such that the p-polarized component of the return radiation impinges upon a first region of the array and the s-polarized component of the return radiation impinges simultaneously upon a second region of the array; and the object lens assembly is further configured to focus the conditioned beam on the object focal plane through a range of incidence angles having (a) altitude angles of 0° to at least about 70° and (b) azimuth angles of 0° to at least about 180°.
 13. The scatterometer of claim 1 wherein: the detector comprises a CMOS imager having an array of pixels and the polarizing beam splitter comprises a cube-type polarizing beam splitter that directs the p- and s-polarized components of the return radiation along parallel paths such that the p-polarized component of the return radiation impinges upon a first region of the array and the s-polarized component of the return radiation impinges simultaneously upon a second region of the array; the object lens assembly is further configured to focus the conditioned beam on the object focal plane through a range of incidence angles having (a) altitude angles of 0° to at least about 70° and (b) azimuth angles of 0° to at least about 180°; and the wavelength is approximately 375 nm to approximately 475 nm.
 14. A method of evaluating a microstructure on a workpiece, comprising: generating a beam having a wavelength; irradiating a microstructure on a workpiece by passing the beam through an object lens assembly that focuses the beam to a focus area at an object focal plane, wherein the beam is focused through at least (a) a 15° range of altitude angles and (b) a 90° range of azimuth angles simultaneously; presenting return radiation scattered from a microstructure on the workpiece in a radiation distribution at a second focal plane; polarizing the radiation distribution using a polarizing beam splitter configured to present separate images of p- and s-polarized components of the return radiation; detecting the separated images of the p- and s-polarized components of the return radiation simultaneously on a single array; and producing a representation of the p- and s-polarized components of the return radiation.
 15. The method of claim 14 wherein the detector comprises a CMOS imager having an array of pixels, and wherein the method further comprises impinging the p-polarized component of the return radiation upon a first region of the array and the s-polarized component of the return radiation upon a second region of the array.
 16. The method of claim 14 wherein the polarizing beam splitter comprises a cube-type polarizing beam splitter, and wherein the method further comprises directing the p- and s-polarized components of the return radiation along parallel paths such that the p-polarized component of the return radiation impinges upon a first region of the array and the s-polarized component of the return radiation impinges simultaneously upon a second region of the array.
 17. The method of claim 14 wherein the object lens assembly is further configured to focus the conditioned beam on the object focal plane through a range of incidence angles having (a) altitude angles of 0° to at least about 70° and (b) azimuth angles of 0° to at least about 180°.
 18. The method of claim 14 wherein the altitude angles are 0° to at least 80° and the azimuth angles are 0° to at least 360°.
 19. The method of claim 14 wherein the wavelength is approximately 200 nm to approximately 475 nm.
 20. The method of claim 14 wherein the wavelength is approximately 375 nm to approximately 475 nm.
 21. The method of claim 14 wherein the first wavelength is approximately 244 nm and the second wavelength is approximately 457 nm.
 22. The method of claim 14 wherein the wavelength is approximately one of 405 nm or 457 nm. 